Ferroelectric smectics have recently entered the domain of liquid crystal applications, in the so-called SSFLC (Surface stabilized ferroelectric liquid crystal) device, as disclosed in U.S. Pat. No. 4,367,924 and characterized, among other things, by a very high switching speed. The device utilizes the chiral smectic C phase or any chiral tilted smectic phase which all have the characteristic of being ferroelectric, i.e. exhibiting a spontaneous macroscopic electric polarization, at least on a local scale.
Due to the presence of a spontaneous polarization, ferroelectric smectics are very sensitive to an external electric field. In the SSFLC device one utilizes the switchability of the permanent polarization vector between two equivalent stable states, differing in the polarization direction ("up" or "down") corresponding to two different directions of tilt, +.theta. and -.theta., where .theta. is the angle between the smectic layer normal and the average direction of the long axes of the molecules. In the device proposed here, we utilize instead a non-tilted and therefore non-ferroelectric smectic phase adjacent to a tilted smectic phase of a chiral material, e.g. the A phase lying directly above the C phase. Due to the electro-clinic effect, first described by S. Garoff and R. B. Meyer in Physical Review Letters, volume 38, page 848, from 1977, there is a response of the optic axis to an applied external electric field. With the smectic layers parallel to the confining glass plates, which has been the experimental condition studied so far, the effect is small and requires phase-sensitive methods for its detection. In very thin samples, 1 to 3 .mu.m, and with the smectic layers perpendicular instead of parallel to the glass plates ( this is the so-called book-shelf geometry typical of the SSFLC cells) the effect is however easily and strikingly detectable at conveniently low applied fields, even in the CMOS-compatible and thus technically accessible range of 10 to 30 volts.
The electro-optic device of the present invention is faster than any liquid crystal device known so far. For the same material ad applied field strength the response is typically one hundred times faster than that of SSFLC devices. Furthermore sample preparation is much easier, due to the wellknown alignment methods that can be used, for instance in the case of the A phase. Because of the absence of ferroelectricity in non-tilted phases the problem of charge-collecting from outside does never arise for the present device.
The bookshelf geometry can be obtained by shearing the glass plates or by other methods, e.g. slow cooling in a magnetic field of about 2 Tesla. The preferred geometry is, however, particularly easily obtained for substances having a nematic phase on top of the smectic A phase. In this case oblique evaporation of SiO, or coating with PVA, polyimide or silane followed by rubbing the surface in a buffing machine, will align the nematic and succeedingly the smectic phase, as is well known by those skilled in the art.